It is well known that good health of turbines are required for its smooth operation and also the monitoring of the condition of turbine is important to maintain uniformity of operation and operative power output from turbines.
In order to assess the turbine health and in particular the turbine blade condition, measurement of turbine blade vibration has been known and followed in the art. Turbines are basically fluid flow machines where moving blades are subject to high loads. In case of any alternating stress it is usual to cause vibration related failures in the blades and in order to ascertain high load condition the measurement of vibration states of the blades have been known to be carried out. Such measurement of the vibration of the blades has been practiced and some of the methods followed in the art are as discussed hereunder.
U.S. Pat. No. 4,996,880 discloses an operating turbine monitor incorporating one or more acoustic sensors positioned inside the turbine next to a blade row of a rotor, an analog digital converter, a tachometer sensor at the rotor shaft, a memory, a synchronous averaging circuit, a sub tractor circuit, and a comparator. The patent was directed to overcoming the problem of monitoring an operating turbine for order related turbine blade vibration and for non order-related turbine blade vibration as well as for other turbine conditions.
U.S. Pat. No. 4,518,917 discloses yet another manner of measurement of vibration of turbine blades. A plurality of proximity sensors are disposed in a turbine flow guide inside the turbine to measure flow guide-to-turbine blade tip distance at various points around the flow guide. The sensor signals are detected and the peak-to-peak voltage of the detected signals provides an indication of the desired distance measurement. The signals are also monitored and if ambient operating conditions causes the signals to exceed predetermined positive or negative thresholds, corrective signals are generated so as to bring the detected signal back into an acceptable range. A proximity sensor is also provided to measure axial movement of the rotor to which the blades are attached so that the other sensor signals can be modified as a function of the measured axial position.
It is also known to provide systems for measurement of blade vibration wherein non-contacting probes are installed inside the turbine to monitor the vibration of tip of the blades in each stage. The probe monitored the displacement of the tip of individual blade and estimated the stress therefrom. However, such non contact measurement of vibration of moving blades presently available are either complex to obtain or are subject to inaccurate measurement of vibration in blades.
U.S. Pat. No. 6,761,528 is directed to such measurement of vibration of moving blades in a non-contact manner and basically relates to steam turbine comprising an optical measuring system for measuring a moving blade vibration. In the said system a transmitter is adapted to produce a light beam which strikes the moving blades and is reflected by these into a receiver. Importantly, as disclosed in said patent by providing the transmitter separate from the receiver the system is stated to achieve reduction of the scattered light effect of the steam to enable a reliable optical measurement of the blade vibration.
It would be apparent from the above that for measurement of blade vibration the systems presently available are either complex or have limitations of inaccurate measurement of blade vibration or in case of the intrusive systems the same is again either very complex or suffer from complexities of unwanted exposure of the sensors/detectors to severe conditions prevailing inside the turbine. Thus, on exposure of such sensors/detectors to such severe conditions inside turbine the systems are prone to failure and malfunctioning of the vibration measurement adding to the problems of reliability of such systems. Moreover, such systems are also not suited to determine the natural frequency of the turbine blades nor can the same serve for the much desired early detection of blade crack.